Analysis of electrooptically induced waveguides in strontium barium niobate and optical modulators

An electrooptically induced optical waveguide can be formed by applying a DC voltage between electrodes fabricated on the surface of a crystal with strong electrooptic coefficients (for example, strontium barium niobate). Calculations of the refractive index profile and the optical mode size for such waveguides are presented. Approximate solutions for the fundamental mode are used to analyse the power coupling between a single-mode step-profile fibre and these waveguides. The power transmission losses in the straight and S-shaped waveguides are estimated by using a finite-difference beam propagation method (FD-BPM). The calculations predict that a new kind of modulator/switch can be realized with the induced waveguides. The important characteristics of these devices are discussed and compared with LiNbO₃ optical waveguide modulators.     

Light modulation by an electrooptic device with feedback

Light modulation by an electrooptic nonlinear device is examined theoretically and experimentally. Very low voltage light modulation may be achieved in a conventional electrooptic polarization modulator when a signal proportional to the optical output power is fed back to the modulator crystal. Very linear light modulation may also be implemented in the other mode of such a device.     

High-performance unidirectional electrooptic modulator based on polymeric highly multi-mode waveguides

A unidirectional electrooptic modulator based on an asymmetrical highly multi-mode waveguide coupler is proposed. Firstly, the energy distributions of all the modes within two highly multi-mode waveguides are analyzed with eigenstate theory. Then, to achieve high switching performance in a guided-wave coupler, a dumping wall is placed on the larger waveguide of an asymmetric pair of waveguides, so that the larger waveguide has a dumping effect. This dumping effect makes the asymmetric highly multi-mode waveguide coupler possess a highly efficient unidirectional coupling process. Due to the large cross-section of the waveguide, a new modulation electrode structure is built. Based on this dumping effect, the unidirectional coupling process is theoretically modeled. Furthermore, in both scenarios, i.e., the unmodulated state and the electrooptically modulated state, the unidirectional coupling efficiencies are studied. As a result, not only can a high unidirectional coupling efficiency of 100% be achieved, but a high electrooptic modulation depth more than 90% can also be implemented in theory, and a high thermooptic modulation depth more than 90% under a low modulation voltage of 30 volts is also experimentally achieved. This device can be used for multi-mode optical interconnection systems such as data communication and fiber sensor networks.     

Adiabatic coupling between conventional dielectric waveguides and waveguides with discrete translational symmetry

We study adiabatic transformation in optical waveguides with discrete translational symmetry. We calculate the reflection and transmission coefficient for a structure consisting of a slab waveguide that is adiabatically transformed into a photonic crystal waveguide and then back into a slab waveguide. The calculation yields high transmission over a wide frequency range of the photonic crystal waveguide band and indicates efficient coupling between the slab waveguide and the photonic crystal waveguide. Other applications of adiabatic mode transformation in photonic crystal waveguides and the coupled-resonator optical waveguides are also discussed.     

基于单通道反射型电光调制器实现不同路光同时调制的方法

提出了一种利用单通道反射型聚合物电光调制器同时调制不同路光的方法。衰减全反射结构的电光调制器,其每一个衰减全反射(ATR)峰的位置分别对应于一个导波共振模式。实验系统中利用衰减全反射导膜峰作为调制通道,使其每一路光路的入射角分别对应于不同导波共振模式的工作角,就可以实现利用单通道的电光调制器同时调制不同路光。提出了三种实现两路光同时调制的模式,并给出了三种模式的调制结果。结果表明,作为调制通道的导模阶数越低,调制效率越高。在832 nm光波波长下,采用最低阶导模进行调制时可以获得42.9%的调制效率。     

基于硅纳米线波导的两级光子晶体缩束器

鉴于在微观领域光波的缩束对实现光电集成的重要意义,提出了基于硅纳米线波导的两级光子晶体缩束器。其中一级压缩基于W5型和W1型光子晶体波导间的高效耦合。二级压缩则由宽为0.1μm,长为3.06μm的纳米线波导和W1型光子晶体波导构成,通过二者的高效耦合实现光束压缩。当W1型光子晶体波导和纳米线波导间介质柱的半径为0.04μm时,对于1550nm波长的电磁波,缩束器的通光效率可达93.4%,压缩比为16.08,出射光束半峰全宽仅为0.148μm。     

Laterally coupled silicon-germanium modulator for passive waveguide systems

We model a laterally coupled Franz-Keldysh add-drop ring modulator designed to overcome the C-band indirect absorption of silicon-germanium. Although our concept is based on loss-sensitive interferometry, it utilizes the same highly absorptive germanium-rich compositions geared toward complementary metal-oxide semiconductor (CMOS) photodetectors and electroabsorption modulators. The proposed device can be integrated with passive waveguide networks in which the carrier plasma modulation mechanism is ineffective. In addition, unlike previous silicon-germanium modulator schemes, complex butt-coupling between the passive transport and the active silicon-germanium waveguides is not required. Instead, the optical mode remains guided within the transport waveguide, minimizing transition losses.     

基于非对称脊波导的多种聚合物弯波导性能分析

以缩短马赫-曾德尔(M-Z)电光调制器分支波导为目的,将有机聚合物非对称结构的脊波导应用于S弯波导中。采用半矢量有限差分光束传输分析法,系统地分析了不同参数下采用非对称脊波导的三种常见弯波导,即正弦弯、圆弧弯和余弦弯分支波导的TM00光场传输损耗,并与采用对称脊波导的结构相比较。研究表明,在芯层厚度h=1.5μm、脊高为0.3μm、脊宽w=4μm、分支高度G=11μm的情况下,当脊波导短边芯层平板宽度s≥2μm时,在相同传输损耗条件下,正弦弯、圆弧弯和余弦弯分支波导长度分别可减少40%、30%和25%,该结果对有机聚合物M-Z调制器中分支波导的设计具有一定的参考价值。     

Composite optical fibre–slab waveguides: a scalar,coupled-mode analysis with vector corrections

Analytically-derived vector corrections are included as additional coupling coefficients in numerical calculations of scalar coupled-mode and compound-mode theory (SCMT) of composite fibre–slab waveguides. The slab waveguide is asymmetric, with moderate light-wave guiding and coupling assumed and the resultant polarization effects are of interest. It is found that the modifications to light propagation and coupling coefficients determined previously by first-order SCMT are dependent mainly on the strength of coupled fibre and slab modes, the thickness of the guiding layers and the ratio of refractive indices of guiding to cladding layer (asymmetry of the slab waveguide). The vector corrections observed are not significant compared with our previous SCMT results, as the optical constants defined appear to satisfy the approximations necessary for the scalar analysis. This revised version was published online in June 2006 with corrections to the Cover Date.     

1.5μm Ti:LiNbO_3光波导TE_0模偏振器

在1.523μm波长He-Ne激光,对于Z切Ti扩散LiNbO_3光波导及其带有金属包覆/介质缓冲层的偏振器结构,理论上计算了单模波导的工艺参数和基模的吸收损耗系数.文中用TM_0模的共振吸收效应,在1.5μm波长首次研制成Ti:LiNbO_3光波导TE_0模偏振器.当器件长度为2mm和9mm时,其消光比分别为20dB和25dB.     

电光调制器的温度特性及其最优化设计

利用线性电光效应的波耦合理论对铌酸锂晶体电光调制器的温度特性进行研究,给出了不同光波波矢和晶体光轴夹角情况下电光效应的温度变化特性,发现可以利用角度调节来克服电光调制器的温度敏感性;在此基础上,进一步对电光调制器进行了最优化设计,得到一个半波电压小(几十伏)、零场泄漏几乎为零(零电压出射光强和入射光强比为0．0027)、消光比达到365．6、温度性能稳定而且不需要透明电极的一个设计。     

Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides

A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.     

Beam propagation in gain–loss balanced waveguides

Linear and nonlinear localized modes of a slab waveguide with distributed gain and loss are studied. The structure is an optical analog of parity-time symmetric potentials in quantum mechanics. Such waveguide structures support stable localized modes. The explicit equation for the propagation constant of linear modes is obtained. The existence of stable nonlinear modes in such waveguides is demonstrated. Bend losses in such structures are analyzed.     

A wide-band polymeric electro-optic modulator array based on unidirectional coupling between multi-mode waveguide array and a vertical configured dumping planar waveguide

A polymeric wide-band electro-optic (EO) modulator array based on unidirectional mode-coupling between the guiding multi-mode waveguide array and a vertical configured planar dumping waveguide was developed. A low insertion loss of <1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. The employment of the dumping planar waveguide not only provides an efficient way to achieve the unidirectional coupling mechanism, but also effectively confines the light coupled from the guiding waveguides within the planar dumping waveguide. The combination of unidirectional coupling and light confinement effects ensures low cross talk of <22 dB between adjacent guiding waveguides. Since the proposed modulator is based on the unidirectional coupling mechanism, it is intrinsically wide-band. A wide-band operation from 1308 to 1324 nm was experimentally obtained. The unidirectional mode-coupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges.     

Novel integrated optic intensity modulator based on mode coupling

Abstract

The analysis, design, realization, and measurements of a novel intensity modulator are reported. The operating principle is based on mode coupling between a passive low-loss SiON waveguide and an electro-optic high-loss polymer waveguiding structure. Matching the waveguides is critical and results in severe demands for the technology. Extended simulations by the Coupled Mode Theory, the Leaky Wave Model, and Finite Difference Beam Propagation Method resulted in the design of several modulator structures. After realization, modulation could be demonstrated at 632 nm and at 1523 nm using lossy waveguiding modes and surface plasmon modes.     

Dispersionless slow light by photonic crystal slab waveguide with innermost elliptical air holes

We report a low-loss photonic crystal slab waveguide formed by deforming the innermost circle air holes in the conventional photonic crystal slab waveguide into elliptical ones. We obtain the photonic bands and group index of guided modes in this photonic crystal waveguide by guided-mode expansion method and investigate the dependence of photonic bands and group index of guided modes on the parameters of the innermost elliptical air holes. The group velocity and group velocity dispersion of this waveguide strongly depend on the innermost elliptical air holes. Photonic crystal slab waveguide with the optimum innermost elliptical air holes possesses a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, the guided mode supported by this waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3-5 nm bandwidth.     

Laser-assisted fabrication of new slab-coupled lithium niobate optical waveguide

Recently a new type of lithium niobate waveguide was suggested for potential nonlinear optic applications. The waveguide consists of a uniform large core and a leaky coupled slab for realizing a lateral optical confinement to support the fundamental spatial mode propagation. Inside the waveguide, the slab layer is required to have a refractive index slightly lower than that of the core, but higher than that of the substrate. Lithium niobate doped with magnesium oxides shows an increased refractive index that is dependent on the dopant's concentration. Therefore, in order to fabricate such waveguides, the pulsed laser deposition approach was used to study the growth of such composition-modified lithium niobate as the slab layer. The as-grown films were characterized on its expitaxy, structure, and optical performance, via X-ray diffraction analysis, optical guiding experiment, etc.     

Numerical studies of 2D photonic crystals: Waveguides,coupling between waveguides and filters

In photonic crystals, light propagation is forbidden in a certain wavelength range, the bandgap. In a two-dimensional crystal composed of parallel high-refractive index rods in a low-index background a line defect can be formed by removing a row of these rods, which can act as a waveguide for frequencies in the bandgap of the crystal. In order to get more insight into the main features of such waveguides we have studied a number of properties, using simulation tools based on the finite difference time domain method and a finite element Helmholtz solver. We show conceptually simple methods for determining the bandgap of the crystal as well as the dispersion of a waveguide for wavelengths in this bandgap. For practical applications, it is also important to know how much light can be coupled into the waveguide. Therefore, the coupling of light from a dielectric slab waveguide into the photonic crystal waveguide has been examined, showing that a coupling efficiency of up to 83% can be obtained between a silicon oxide slab and a waveguide in a crystal of silicon rods. Finally, calculations on an ultra-compact filter based on reflectively terminated side-branches of waveguides (similar to tuned stubs in microwave engineering) are shown and discussed.     

利用石英晶体的线性电光和电致旋光强度调制

提出并实验研究了利用石英晶体自身特性产生光学偏置的线性电光调制器。利用某些电光晶体的自然旋光性和自然双折射,并合理设计晶体尺寸及其晶轴与光传播方向之间的夹角,可以提供所需要的光学偏置以实现大动态范围的线性电光调制。对于兼有电光和电致旋光效应的晶体,例如石英晶体,应考虑综合利用这两种效应对电光调制的贡献。实验研究了一块石英晶体的电光强度调制特性,在27V～4.5kV工频调制电压范围内,调制输出信号与调制电压之间的线性相关系数大于0.9999。     

Novel integrated optic intensity modulator based on mode coupling

The analysis, design, realization, and measurements of a novel intensity modulator are reported. The operating principle is based on mode coupling between a passive low-loss SiON waveguide and an electro-optic high-loss polymer waveguiding structure. Matching the waveguides is critical and results in severe demands for the technology. Extended simulations by the Coupled Mode Theory, the Leaky Wave Model, and Finite Difference Beam Propagation Method resulted in the design of several modulator structures. After realization, modulation could be demonstrated at 632 nm and at 1523 nm using lossy waveguiding modes and surface plasmon modes.